uniform sampler2D dst_texture;+
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uniform sampler2D mask_texture;+
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uniform vec2 inv_mask_size;+
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uniform vec2 inv_dst_size;+
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uniform vec2 mask_offset;+
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uniform vec4 mask_channel;+
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+
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float mask()+
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{+
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    return dot(mask_channel, texture2D(mask_texture, (gl_FragCoord.xy + mask_offset) * inv_mask_size));+
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}+
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+
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void main()+
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{+
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    vec4 dst = texture2D(dst_texture, gl_FragCoord.xy * inv_dst_size);+
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+
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    // combine clip and coverage channels+
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    float mask_alpha = mask();+
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+
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    gl_FragColor = mix(dst, composite(brush(), dst), mask_alpha);+
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}+
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